The present invention relates to a method of and apparatus for measuring dampening water for a printing machine. More particularly, the invention is concerned with a method of and apparatus for optically measuring the amount of dampening water on the surface of a plate attached to a plate cylinder of an offset press for the purpose of controlling the rate of supply of the dampening water to the plate surface.
Hitherto, various methods have been proposed for measuring the amount of dampening water on the plate surface. For instance, it is known to measure the amount of the dampening water through measuring the amount of absorption of infrared rays by the dampening water. According to another known method, the amount of dampening water is detected through measurement of the humidity around the plate surface. According to still another known method, the amount of dampening water is determinined by measuring the electric resistance of the water film on a dampening water roller. These methods, however, have not been put into practical use yet.
French Patent No. 2,104,478 discloses a method which makes use of a phenomenon that, when parallel rays of light are applied to a thin film of water, the intensity of the reflected light in relation to the reflecting angle varies depending on the thickness of the thin water film. Namely, the above-mentioned French patent proposes to measure the amount of the dampening water by applying light to the plate surface and measuring the quantity of the light reflected from the plate surface. This measuring method proposed by the above-mentioned French patent involves the following problems. The apparatus for measuring the light reflected by the plate surface has a light-receiving element which is disposed in the angular region of, for example, 60.degree. to 80.degree. which is smaller than the angle for mirror-reflection which is equivalent to the incident angle of the light applied to the plate surface; so as to receive and measure the diffused light. Therefore, the light-receiving element can receive only small quantity of reflected light and, hence, often suffers a large error in the measurement. Therefore, as will be fully explained later, the quantity of the received light cannot be determined by a single-valued function in relation to the quantity of light received. Furthermore, since the angle of incidence of the light is as large as 80.degree. to 85.degree., the incoming light is interferred by the roughness of the plate surface so that the range of measurement is limited undesirably.